Lithographic apparatus and device manufacturing method

ABSTRACT

In a lithographic apparatus, a localized area of the substrate surface under a projection system is immersed in liquid. The height of a liquid supply system above the surface of the substrate can be varied using actuators. A control system uses feedforward or feedback control with input of the surface height of the substrate to maintain the liquid supply system at a predetermined height above the surface of the substrate.

This application is a continuation of U.S. patent application Ser. No.13/082,038, filed Apr. 7, 2011 now U.S. Pat. No. 8,724,083, which is acontinuation of U.S. patent application Ser. No. 12/068,546, filed Feb.7, 2008, now U.S. Pat. No. 7,936,444, which is a continuation of U.S.patent application Ser. No. 10/844,575, filed May 13, 2004, now U.S.Patent No. 7,352,434, which claims priority to European PatentApplications 03252955.4 and 03256643.2, filed May 13, 2003 and Oct. 22,2003, respectively, the entire contents of each of the foregoingapplications incorporated herein by reference.

BACKGROUND

1. Field

The present invention relates to a lithographic projection apparatus anda device manufacturing method.

2. Description of the Related Art

A lithographic apparatus is a machine that applies a desired patternonto a target portion of a substrate. Lithographic apparatus can beused, for example, in the manufacture of integrated circuits (ICs). Inthat circumstance, a patterning device, such as a mask, may be used togenerate a circuit pattern corresponding to an individual layer of theIC, and this pattern can be imaged onto a target portion (e.g. includingpart of one, or several, dies) on a substrate (e.g. a silicon wafer)that has a layer of radiation-sensitive material (resist). In general, asingle substrate will contain a network of adjacent target portions thatare successively exposed. Known lithographic apparatus include so-calledsteppers, in which each target portion is irradiated by exposing anentire pattern onto the target portion at once, and so-called scanners,in which each target portion is irradiated by scanning the patternthrough the beam in a given direction (the “scanning”-direction) whilesynchronously scanning the substrate parallel or anti-parallel to thisdirection.

It has been proposed to immerse the substrate in the lithographicprojection apparatus in a liquid having a relatively high refractiveindex, e.g. water, so as to fill a space between the final element ofthe projection system and the substrate. The point of this is to enableimaging of smaller features since the exposure radiation will have ashorter wavelength in the liquid. (The effect of the liquid may also beregarded as increasing the effective NA of the system and alsoincreasing the depth of focus.)

However, submersing the substrate or substrate and substrate table in abath of liquid, as disclosed for example in U.S. Pat. No. 4,509,852,incorporated herein by reference, means that there is a large body ofliquid that to be accelerated during a scanning exposure. This requiresadditional and/or more powerful motors and turbulence in the liquid maylead to undesirable and unpredictable effects.

One of the solutions proposed is for a liquid supply system to provideliquid on only a localized area of the substrate and in between thefinal element of the projection system and the substrate using a liquidconfinement system as the substrate generally has a larger surface areathan the final element of the projection system. One way which has beenproposed to arrange for this is disclosed in WO 99/49504, incorporatedherein by reference. As illustrated in FIGS. 2 and 3, liquid is suppliedby at least one inlet IN onto the substrate, preferably along thedirection of movement of the substrate relative to the final element,and is removed by at least one outlet OUT after having passed under theprojection system. That is, as the substrate is scanned beneath theelement in the −X direction, liquid is supplied at the +X side of theelement and taken up at the

−X side. FIG. 2 shows the arrangement schematically in which liquid issupplied via inlet IN and is taken up on the other side of the finalelement of the projection system PL by outlet OUT which is connected toa low pressure source. In the illustration of FIG. 2 the liquid issupplied along the direction of movement of the substrate relative tothe final element, though this does not need to be the case. Variousorientations and numbers of inlets and outlets positioned around thefinal element are possible. One example is illustrated in FIG. 3 inwhich four sets of an inlet with an outlet on either side are providedin a regular pattern around the final element of the projection systemPL.

Another solution which has been proposed is to provide the liquid supplysystem with a seal member which extends along at least a part of aboundary of the space between the final element of the projection systemand the substrate table. The seal member is substantially stationaryrelative to the projection system in the XY plane, though there may besome relative movement in the Z direction (i.e. the direction of theoptical axis). A seal is formed between the seal member and the surfaceof the substrate. The seal may be a contactless seal such as a gas seal.

If the substrate is immersed in liquid as proposed, some residual liquidcan remain on the surface of the substrate after it has been exposed bythe projection system. This liquid can cause problems in subsequentprocessing of the substrate.

SUMMARY

It is an aspect of the present invention to reduce the residual liquidleft on the surface of a substrate after exposure by a projectionsystem.

According to an aspect of the invention, there is provided alithographic projection apparatus including a radiation systemconfigured to provide a beam of radiation; a support configured tosupport a patterning device, the patterning devices configured topattern the beam according to a desired pattern; a substrate tableconfigured to hold a substrate; a projection system configured toproject the patterned beam onto a target portion of the substrate andhaving an optical axis; and a liquid supply system configured to providean immersion liquid on the substrate in a space between a final elementof the projection system and the substrate, wherein at least part of theliquid supply system is free to at least one of move in the direction ofthe optical axis and rotate about at least one axis perpendicular to theoptical axis.

The liquid supply system can therefore move relative to the surface ofthe substrate to accommodate changes in the surface height of thesubstrate without requiring a large clearance between the supply systemand the substrate surface. The entire supply system, or only those partslikely to come into contact with the surface of the substrate, such as asealing member, may be moved. This is particularly useful when using aliquid supply system which provides liquid to only a localized area ofthe substrate. Also, the liquid supply system can be moved away from thesubstrate, for instance during a TIS scan, in the Z direction androtated about axes parallel to the X and Y directions.

The apparatus may further include an actuator configured to adjust theheight and/or tilt of at least part of the liquid supply system relativeto the substrate. This allows the height and/or tilt of the liquidsupply system to be altered as needed.

The apparatus may further include a control system configured to controlthe actuation means to maintain a predetermined height of the liquidsupply system above the substrate. This ensures that the height of theliquid supply is maintained at a desired height. The height can bechosen to minimize the residue of the liquid left on the substrate as itis scanned under the projection system.

In one embodiment, the apparatus may further include at least one sensorconfigured to measure a height of at least part of the liquid supplysystem above the surface of the substrate, and the control system uses afeedback control method with input from the at least one sensor. Thefeedback control method allows the height to be controlled accuratelybased on the actual surface height of the substrate as it is scannedunder the projection system.

In another embodiment, the apparatus may further include a measurementsystem configured to measure a surface height of the substrate prior tothe entry of the substrate into the projection system and to store themeasured height in a storage device, and the control system usesfeedforward control with input of the measured height from the storagedevice. If the surface height of the substrate is know prior to scanningof the substrate under the exposure system, this data can be used forfeedforward control of the height of the liquid supply system.

In another embodiment the apparatus may further include at least onesensor configured to measure a height of the substrate in an exposureposition, and the control system uses a feedforward control method withinput of the height of the substrate in an exposure position. Theapparatus can then measure the height of the substrate as it is exposedby the projection system. This measurement can then be used as afeedforward input for when that part of the substrate passes under theliquid supply system. Alternatively a feedback control method can beemployed.

In the non-actuated state, the actuator may position the liquid supplysystem to its maximum setting away from the surface of the substrate inthe direction of the optical axis of the projection system. This allowsthe control system to fail-safe. If the control signal is not suppliedto the actuator (i.e. it is in a non-actuated state) the supply systemis not in danger of colliding with the substrate as the supply system ispositioned as far away from the substrate's surface as possible, but notso far so that immersion liquid escapes between the liquid supply systemand the substrate.

The actuator may be part of the liquid supply system, the actuatorincluding a seal extending along at least part of the boundary of thespace between the final element of the projection system and thesubstrate table; and a gas seal configured to form a gas seal betweenthe seal member and the surface of the substrate. The pressure in thegas seal is variable to adjust the height and/or tilt of the liquidsupply system relative to the substrate. The gas seal acts to retain theliquid within the desired space and will also reduce the residue ofliquid left on the substrate after it has been scanned under theprojection system. The gas seal is also used to adjust the height of theliquid supply system, thereby simplifying construction as no dedicatedactuator is needed.

The actuator may be connected between the liquid supply system and thebase frame of the apparatus. Alternatively, the actuator may beconnected between the liquid supply system and the reference frame ofthe apparatus. The reference frame supports, inter alia, the projectionsystem.

The predetermined height may be in the range of 10 μm to 1000 μm. If theheight is in the mentioned range the residual liquid remaining on thesubstrate after scanning is reduced. The height can also be variedaccording to the viscosity of the immersion liquid, or increased ordecreased to increase/decrease the amount of liquid filling the space.

According to a further aspect of the invention there is provided adevice manufacturing method including providing a substrate that is atleast partially covered by a layer of radiation-sensitive material;projecting a patterned beam of radiation onto a target portion of thelayer of radiation-sensitive material using a projection system;providing a liquid on the substrate to fill a space between thesubstrate and a final element of the projection system; and allowing asystem which provides the liquid to move freely in the direction of theoptical axis of the projection system.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,liquid-crystal displays (LCDs), thin-film magnetic heads, etc. It shouldbe appreciated that, in the context of such alternative applications,any use of the terms “wafer” or “die” herein may be considered assynonymous with the more general terms “substrate” or “target portion”,respectively. The substrate referred to herein may be processed, beforeor after exposure, in for example a track (a tool that typically appliesa layer of resist to a substrate and develops the exposed resist) or ametrology or inspection tool. Where applicable, the disclosure hereinmay be applied to such and other substrate processing tools. Further,the substrate may be processed more than once, for example in order tocreate a multi-layer IC, so that the term substrate used herein may alsorefer to a substrate that already contains multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of 365, 248, 193, 157 or 126 nm).

The term “patterning devices” used herein should be broadly interpretedas referring to a device that can be used to impart a beam of radiationwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the projection beam may not exactly correspond to thedesired pattern in the target portion of the substrate. Generally, thepattern imparted to the projection beam will correspond to a particularfunctional layer in a device being created in the target portion, suchas an integrated circuit.

Patterning devices may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. In this manner, thereflected beam is patterned. In each example of a patterning device, thesupport may be a frame or table, for example, which may be fixed ormovable as needed and which may ensure that the patterning device is ata desired position, for example with respect to the projection system.Any use of the terms “reticle” or “mask” herein may be consideredsynonymous with the more general term “patterning devices” or“patterning structures.”

The term “projection system” used herein should be broadly interpretedas encompassing various types of projection system, including refractiveoptical systems, reflective optical systems, and catadioptric opticalsystems, as appropriate for example for the exposure radiation beingused, or for other factors such as the use of an immersion fluid or theuse of a vacuum. Any use of the term “lens” herein may be considered assynonymous with the more general term “projection system”.

The illumination system may also encompass various types of opticalcomponents, including refractive, reflective, and catadioptric opticalcomponents to direct, shape, and/or control the beam of radiation, andsuch components may also be referred to below, collectively orsingularly, as a “lens”.

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic projection apparatus according to anembodiment of the invention;

FIG. 2 depicts a side view of a liquid supply system according to theprior art;

FIG. 3 depicts a plan view of the proposed liquid supply system shown inFIG. 2;

FIG. 4 depicts the liquid reservoir of an embodiment of the invention;

FIG. 5 is an enlarged view of part of the liquid reservoir of FIG. 4;

FIG. 6 depicts the liquid reservoir of another embodiment of theinvention;

FIG. 7 is an enlarged view of part of the liquid reservoir of FIG. 6;

FIG. 8 depicts the liquid reservoir of another embodiment of theinvention; and

FIG. 9 depicts the control of the liquid supply system and substratetable.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus including anillumination system (illuminator) IL configured to provide a beam PB ofradiation (e.g. UV radiation). A first support (e.g. a mask table) MT isconfigured to support a patterning device (e.g. a mask) MA and isconnected to a first positioning device PM that accurately positions thepatterning device with respect to a projection system PL. A substratetable (e.g. a wafer table) WT is configured to hold a substrate (e.g. aresist-coated wafer) W and is connected to a second positioning devicePW that accurately positions the substrate with respect to theprojection system PL. The projection system (e.g. a refractiveprojection lens) PL is configured to image a pattern imparted to thebeam PB by the patterning device MA onto a target portion C (e.g.including one or more dies) of the substrate W.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above).

The illuminator IL receives radiation from a radiation source LA. Thesource and the lithographic apparatus may be separate entities, forexample when the source is an excimer laser. In such cases, the sourceis not considered to form part of the lithographic apparatus and theradiation beam is passed from the source LA to the illuminator IL withthe aid of a beam delivery system including, for example, suitabledirecting mirrors and/or a beam expander EX. In other cases the sourceLA may be an integral part of the apparatus, for example when the sourceis a mercury lamp. The source LA and the illuminator IL, together withthe beam delivery system if needed, may be referred to as a radiationsystem.

The illuminator IL may include an adjusting device(s) AM configured toadjust the angular intensity distribution of the beam. Generally, atleast the outer and/or inner radial extent (commonly referred to asσ-outer and σ-inner, respectively) of the intensity distribution in apupil plane of the illuminator can be adjusted. In addition, theilluminator IL generally includes various other components, such as anintegrator IN and a condenser CO. The illuminator provides a conditionedbeam of radiation PB having a desired uniformity and intensitydistribution in its cross-section.

The beam PB is incident on a patterning device, illustrated in the formof the mask MA, which is held on the mask table MT. Having traversed themask MA, the projection beam PB passes through the projection system PL,which focuses the beam onto a target portion C of the substrate W. Withthe aid of the second positioning device PW and a position sensor IF(e.g. an interferometer), the substrate table WT can be Moved accuratelyto position different target portions C in the path of the beam PB.Similarly, the first positioning device PM and another position sensor(e.g. an interferometer, not depicted in FIG. 1) can be used toaccurately position the mask MA with respect to the path of the beam PB,e.g. after mechanical retrieval from a mask library, or during a scan.In general, movement of the object tables MT and WT will be realizedwith the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which form part of thepositioning devices PM and PW. However, in the case of a stepper, asopposed to a scanner, the mask table MT may be connected to a shortstroke actuator only, or may be fixed. The mask MA and the substrate Wmay be aligned using mask alignment marks M1, M2 and substrate alignmentmarks P1, P2.

The depicted apparatus can be used in the following modes:

1. In step mode, the mask table MT and the substrate table WT are keptessentially stationary, while an entire pattern imparted to theprojection beam is projected onto a target portion C at once (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the mask table MT and the substrate table WT arescanned synchronously while a pattern imparted to the projection beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the masktable MT is determined by the (de-)magnification and image reversalcharacteristics of the projection system PL. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the mask table MT is kept essentially stationaryholding a programmable patterning device, and the substrate table WT ismoved or scanned while a pattern imparted to the beam is projected ontoa target portion C. In this mode, generally a pulsed radiation source isemployed and the programmable patterning device is updated as neededafter each movement of the substrate table WT or in between successiveradiation pulses during a scan. This mode of operation can be readilyapplied to maskless lithography that utilizes programmable patterningdevices, such as a programmable mirror array of a type as referred toabove.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

FIG. 4 shows a liquid reservoir 10 between the projection system PL andthe substrate table WT, which is described in detail in European PatentApplication No. 03252955.4. The liquid reservoir 10 is filled with aliquid 11 having a relatively high refractive index, e.g. water,provided via inlet/outlet ducts 13. The liquid has the effect that theradiation of the beam has a shorter wavelength in the liquid than in airor a vacuum, allowing smaller features to be resolved. It is well knownthat the resolution limit of a projection system is determined, interalia, by the wavelength of the beam and the numerical aperture NA of thesystem. The presence of the liquid may also be regarded as increasingthe effective numerical aperture. Furthermore, at fixed numericalaperture, the liquid is effective to increase the depth of focus.

The reservoir 10 forms a contactless seal to the substrate around theimage field of the projection system so that liquid is confined to filla space between the substrate surface and a final element of theprojection system. The reservoir is formed by a seal 12 positioned belowand surrounding the final element of the projection system PL. Liquid isbrought into the space below the projection system and within the seal12. The seal 12 extends a little above the final element of theprojection system and the liquid level rises above the final element sothat a buffer of liquid is provided. The seal 12 has an inner peripherythat at the upper end closely conforms to the step of the projectionsystem or the final element thereof and may be round, for example. Atthe bottom, the inner periphery closely conforms to the shape of theimage field, for example rectangular, though this need not be the case.

The liquid is confined in the reservoir by a gas seal 16 between thebottom of the seal member 12 and the surface of the substrate W. The gasseal is formed by gas, for example air or synthetic air, but may be N₂or another inert gas, provided under pressure via an inlet 15 to the gapbetween the seal 12 and the substrate W and extracted via a first outlet14. The overpressure on the gas inlet 15, vacuum level on the firstoutlet 14 and geometry of the gap are arranged so that there is ahigh-velocity gas flow inwards that confines the liquid. This is shownin more detail in FIG. 5.

The gas seal is formed by two annular grooves 18, 19 which are connectedto the first inlet 15 and the first outlet 14, respectively, by a seriesof small conducts spaced around the grooves. A large annular hollow inthe seal 12 may be provided in each of the inlet 15 and the outlet 14 toform a manifold. The gas seal 16 may also be effective to support theseal 12 by behaving as a gas bearing.

A gap G1, on the outer side of the gas inlet 15, may be small and longso as to provide resistance to air flow outwards. A gap G2, at theradius of the inlet 15, is a little larger to ensure a sufficientdistribution of gas around the seal 12, the inlet 15 being formed by anumber of small holes around the seal 12.

A gap G3 is chosen to control the gas flow through the seal 12. A gap G4is larger to provide a good distribution of vacuum, the outlet 14 beingformed of a number of small holes in the same manner as the inlet 15. Agap G5 is small to prevent gas/oxygen diffusion into the liquid in thespace, to prevent a large volume of liquid entering and disturbing thevacuum and to ensure that capillary action will always fill it withliquid.

The gas seal 16 is thus a balance between the capillary forces pullingliquid into the gap and the airflow pushing liquid out. As the gapwidens from G5 to G4, the capillary forces decrease and the airflowincreases so that the liquid boundary will lie in this region and bestable even as the substrate moves under the projection system PL.

The pressure difference between the inlet 15 at G2 and the outlet 14 atG4, as well as the size and geometry of gap G3, determine the gas flowthrough the gas seal 16 and will be determined according to the specificembodiment. However, if the length of gap G3 is short and absolutepressure at G2 is twice that at G4, the gas velocity will be the speedof sound in the gas and cannot rise any higher. A stable gas flow willtherefore be achieved.

The gas outlet system can also be used to completely remove the liquidfrom the system by reducing the gas inlet pressure and allowing theliquid to enter gap G4 and be sucked out by a vacuum system, which caneasily be arranged to handle the liquid, as well as the gas used to formthe seal. Control of the pressure in the gas seal can also be used toensure a flow of liquid through gap G5 so that liquid in this gap thatis heated by friction as the substrate moves does not disturb thetemperature of the liquid in the space below the projection system.

The shape of the seal 12 around the gas inlet 15 and outlet 14 should bechosen to provide laminar flow as far as possible so as to reduceturbulence and vibration. Also, the gas flow should be arranged so thatthe change in flow direction at the liquid interface is as large aspossible to provide maximum force confining the liquid.

The liquid supply system circulates liquid in the reservoir 10 so thatfresh liquid is provided to the reservoir 10.

The gas seal 16 can produce a force large enough to support the seal 12.Indeed, it may be possible to bias the seal 12 towards the substrate tomake the effective weight supported by the seal 12 higher. The seal 12will in any case be held in the XY plane (perpendicular to the opticalaxis) in a substantially stationary position relative to and under theprojection system but decoupled from the projection system. The seal 12is free to move in the Z direction and can therefore move to accommodatechanges in the surface height of the substrate.

When the substrate W is being moved shearing forces will try to move thepenetration level of the liquid in the gap between the liquid supplysystem and the substrate either to the outside or to the inside (left orright as illustrated). Both are unwanted, to the outside may lead toleakage, and to the inside may lead to air bubbles in the liquid. Thiscan also happen as the height of the liquid supply system varies. Oneway to keep the liquid meniscus in a constant position is to monitor andactively control the position of liquid under the liquid supply system.The control may be implemented by locally increasing and decreasing theair and vacuum pressures in the gas seal 16.

The monitoring can be done in several ways. One way is by measuring thecapacitance between neighbouring metal plates mounted on the bottom ofthe liquid supply system or by measuring the capacitance between such aplate and the substrate or substrate table. Another way is by measuringthe magnetic properties of the medium, be it air or liquid. Since boththe electrical as well as magnetic signals will scale with the liquidposition an accurate positional measurement is possible.

When a conducting liquid like water is used, the conducting propertiesof the liquid can be used by having electrical contacts which are openedor closed. A minimum of two pairs of contacts are needed, one should beopen and one should be closed. Sensing of closure or opening of thecontacts will lead respectively to an increase or decrease in the airpressure of the gas seal or respectively decrease and increase theunderpressure of the vacuum. If a smoother control is needed the numberof contacts can be increased.

Alternatively the effects of these shearing forces can be mitigated byadjusting the height and tilt of the seal member 12 as described herein.Also, it can be predicted that a height adjustment of the liquid supplysystem will induce movement of the meniscus and the pressure in the sealcan be adjusted in a feedforward manner to account for this.

Referring to FIGS. 6 and 7, a second gas outlet 216 is provided on theopposite side of the gas inlet 15 to the first gas outlet 14. In thisway any gas escaping from the gas inlet 15 outwards away from theoptical axis of the apparatus is sucked up by second gas outlet 216which is connected to a vacuum source. In this way gas is prevented fromescaping from the gas seal so that it cannot interfere, for example,with interferometer readings or with a vacuum in which the projectionsystem and/or substrate are housed.

Using the two gas outlet embodiment is also similar to the design of airbearings previously used in lithographic projection apparatus. Thus theexperience gained with those air bearings can be applied directly to thegas seal of the invention. The gas seal of FIGS. 6 and 7 is particularlysuitable for use as gas bearing, as well as a gas seal, such that it canbe used to support the weight of the seal 12.

Sensors may be provided to either measure the distance between thebottom face of the seal 12 and the substrate W or the topography of thetop surface of the substrate W. The sensors can be pneumatic,capacitive, optical (such as a level sensor or interferometer),electrical, magnetic, and/or a combination of the foregoing or any othersensor. A controller may then be used to vary the pressures applied tothe gas inlet 15 and gas outlets 14, 216 to vary the pressure P2 thatconstrains the liquid 11 in the reservoir and the pressures P1 and P3that support the seal 12. Thus the distance D between the seal 12 andthe substrate W may be varied or kept at a constant distance. The samecontroller may be used to keep the seal 12 level. The controller may becontrolled either by a feedforward or a feedback control loop. In afeedforward control system the measured topography of the top surface ofthe substrate is supplied as an input. The measurement may be takenplace in a separate measurement prior to the immersion of the substratein the projection system, or can take place when as the image isprojected to the target portion of the substrate. In a feedback controlsystem a sensor measures the distance between the seal 12 and the topsurface of the substrate, this then forms the input to the controlsystem.

Furthermore, the height of the liquid supply system above the substratecan be calculated from a knowledge of the position of the substratetable WT, the levelling map of the substrate made during the measurementand the height of the liquid supply system relative to the projectionsystem PL, the metrology reference frame RF or the base frame BF.

FIG. 7 shows in detail how the gas seal 16 can be regulated to controlindependently the pressure P2 that holds the liquid 11 in the reservoirand the pressure P3 that supports the seal member 12. This controlminimizes liquid losses during operation, and hence the liquid residueremaining on the substrate after scanning. The pressures P2 and P3 canalso be controlled independently to account for varying conditionsduring exposure. Varying conditions might be different levels of liquidloss per unit time because of different scanning speeds or perhapsbecause the edge of a substrate W is being overlapped by the seal 12.This is achieved by varying the distance to the substrate W of discreteportions of the face of the seal 12 facing the substrate W. Theseportions include the portion 220 between the first gas outlet 14 and theedge of the seal 12 nearest the optical axis, the portion 230 betweenthe gas inlet 15 and the first gas outlet 14 and the portion 240 betweenthe second gas outlet 216 and the gas inlet 15. These portions may bemoved towards and away from the substrate W by piezoelectric actuators,for example. That is the bottom face of the seal 12 may includepiezoelectric actuators (e.g. stacks) which can be expanded/contractedby the application of a potential difference across them. Othermechanical devices could also be used.

The pressure P3 that is created below the gas inlet 15 is determined bythe pressure P % of gas applied to the gas inlet 15, the pressures P6,P4 of gas applied to the first and second gas outlets 14, 216,respectively, and by the distance D between the substrate W and thebottom face of the seal 12 facing the substrate W. Also the horizontaldistance between the gas inlet 15 and outlets 14, 216 has an effect.

The weight of the seal 12 is compensated for by the pressure P3 so thatthe seal 12 settles a distance D from the wafer W. A decrease in thedistance D leads to an increase in the pressure P3 and an increase inthe distance D will lead to a decrease in the pressure P3. Thereforethis is a self regulating system.

The distance D, at a constant pushing force due to the pressure P3, canonly be regulated by the pressures P4, P5 and P6. However, thecombination of the pressures P5, P6 and the distance D creates thepressure P2, which is the pressure keeping the liquid 11 in thereservoir. The amount of liquid escaping from a liquid container atgiven levels of pressure can be calculated and the pressure in theliquid P_(LIQ) is also a factor. If P_(LIQ) is larger than P2, theliquid escapes from the reservoir and if P_(LIQ) is less than P2, airbubbles will occur in the liquid, which is undesirable. It is desirableto try to maintain the pressure P2 at a value slightly less than P_(LIQ)to ensure that no bubbles form in the liquid, and also to ensure thatnot too much liquid escapes as this liquid needs to be replaced. Thismay all be done with a constant distance D. If the distance D1 betweenportion 220 and the wafer W is varied, the amount of liquid escapingfrom the reservoir can be varied considerably as the amount of liquidescaping varies as a square of the distance D1. The variation indistance required is only of the order of 1 mm and this can easily beprovided by a piezoelectric stack with an operational voltage of theorder of 100 V or more.

Alternatively, the amount of liquid which can escape can be regulated byplacing a piezoelectric element at the bottom of the portion 230.Changing the distance D2 is effective to change the pressure P2.However, this solution might require adjustment of the pressure P5 inthe gas inlet 15 in order to keep the distance D constant.

The piezoelectric elements are connected so that when no control signalis applied to them, the supply member is positioned above the substrate.This allows the chance of damage in the event of a malfunction to bereduced; when no signal is supplied the seal 12 is positioned above thesubstrate surface and so cannot collide with it.

The distance D3 between the lower part of the portion 240 and thesubstrate W can also be varied in a similar way and can be used toregulate independently the pressures P2 and P3. It should be appreciatedthat the pressures P4, P5 and P6 and the distances D1, D2 and D3 can allbe regulated independently or in combination to achieve the desiredvariation of the pressures P2 and P3.

The apparatus shown in FIGS. 6 and 7 is effective for use in activemanagement of the quantity of liquid in the reservoir 10. The standbysituation of the apparatus could be, where no substrate W is beingimaged, that the reservoir 10 is empty of liquid but that the gas sealis active thereby to support the seal member 12.

After the substrate W has been positioned, liquid is introduced into thereservoir 10. The substrate W is then imaged. Before the substrate W isremoved the liquid from the reservoir can be removed. After exposure ofthe last substrate the liquid in the reservoir 10 will be removed.Whenever liquid is removed, a gas purge has to be applied to dry thearea previously occupied by liquid. The liquid can be removed easily inthe apparatus by variation of the pressure P2 while maintaining thepressure P3 constant as described above. In other embodiments a similareffect can be achieved by varying the pressures P5 and P6 (and thepressure P4, if necessary or applicable).

Referring to FIG. 8, liquid is supplied by at least one inlet IN andremoved by at least one outlet OUT which make up a liquid confinementsystem. The liquid is supplied and taken up in the same direction as thescanning direction of the substrate. The liquid supply and take-upsystem 310, is positioned horizontally in the X-Y plane, parallel to thesurface of the substrate, by supporting members 312 which connect thesupply and take-up system 310 to a base frame BF. Supporting members 312may be actuators so that if the projection system moves in the X-Yplane, the liquid supply system can be held substantially stationaryrelative to the projection system PL in the X-Y plane. A further set ofactuators 314 connected between the liquid supply and take-up system 310and a reference frame RF that also supports the projection system PL.The actuators 314 control the position of the supply and take-up system310 in the Z direction, parallel to the optical axis of the projectionsystem PL. However, the liquid supply system could be attached to onlyone of the reference RF and base BF frames, or both, and the functionsof the connections to those frames could be reversed from what isdescribed above. The actuators 314 can be piezoelectric, Lorentz motors,excenter mechanisms, linear (electrical, magnetic or a combination) orother actuators. In the situation where no signal is supplied to theactuators, the supply and take-up system 310 is positioned above thesubstrate to reduce the risk of collision. A signal is supplied to theactuators to move the supply and take-up system 310 closer to thesurface of the substrate. The verticle movement possible is desirably ofthe order of several hundred microns.

In use a feedforward or feedback control system (as described above)controls the actuators 314 to maintain the liquid supply and take-upsystem 310 at a predetermined height above the surface of the substrate.This enables the clearance to be small if desired, enabling the liquidresidue remaining on the substrate after scanning to be reduced withoutincreasing the risk of collision.

The actuators 314 can also be connected between the supply and take-upsystem 310 and the projection system PL or the base frame BF. Theactuators can also act in combination with the pneumatic orpiezoelectric systems described above with respect to FIGS. 4-7.

The vertical positioning system shown in FIG. 8 can also be used toposition the seal 12 of the liquid confinement systems described abovewith respect to FIGS. 4-7. In that case, it is not necessary to have theinlet 15 and the seal between the seal 12 and the substrate W can beprovided by vacuum through the outlet 14 only. However, providing an airflow through the inlet 15 can be used as a safety feature to provide anair cushion between the liquid supply system and the substrate. In thiscase it may be useful to have sensors 20 positioned on the seal 12 in abottom surface of the seal 12 radially outwardly of the gas seal 16. Thesensors may be an air gage, or a capacitive sensor, etc. As with theapparatus of FIG. 8, it is also possible to measure the difference inthe distance between the liquid supply system and the reference frame RFor the base frame BF and the substrate table WT and the same frame.

An embodiment with the seal 12 where no gas seal 16 is present betweenthe seal 12 and the substrate W is also possible. In this case liquid isallowed to leak between the seal member 12 and the substrate W. Such aseal member is disclosed, for example, in commonly assigned, co-pendingU.S. application Ser. No. 10/743,271, filed Dec. 23, 2003, incorporatedherein by reference.

The present invention cannot only be used to maintain the distancebetween the liquid supply system and the substrate but can also be usedto move the liquid supply system out of the way during substrate swap.This is possible by use of a closure disk 20, as shown in FIG. 4, inwhich a disk 20 is placed under the projection system to act as a dummysubstrate so that the liquid supply system does not need to be switchedoff during substrate swap. Such a system is disclosed in European PatentApplication 03254059.3, incorporated herein by reference. In this waythe liquid supply system may be moved away from the substrate table WTduring substrate swap thereby reducing cycle time.

The following description assumes that the height of the liquid supplysystem above the substrate table WT is measured by comparing thedistance of the liquid supply system to the metrology reference frame MTwith the distance of the substrate table WT from the metrology referenceframe MT. However, the same control program can be used if the height ofthe liquid supply system above the substrate table WT is measureddirectly or if the height is measured indirectly by reference to anyother point or part of the apparatus.

One of the greatest hazards of an immersion lithography machine islosing control of the machine resulting in collision between the liquidsupply system and the substrate or the substrate table, particularly ifTIS sensors or positioning mirror blocks are on the table WT as they canbe scratched by collision with the liquid supply system. In order toalleviate this hazard, it is proposed to continuously monitor the gapbetween the liquid supply system and the substrate table WT, asmentioned above. This position signal is differentiated to obtain arelative velocity signal.

The liquid supply system and substrate table WT geometries are arrangedsuch that in its uppermost position the liquid supply system cannotcollide with the substrate table in its uppermost position. Conversely,in the lowest position the liquid supply system can obtain, thesubstrate table WT can be moved to an even lower position wherecollision with the liquid supply system cannot occur. Furthermore, theactuators of the substrate table are arranged so that a largeracceleration of the substrate table WT downwards can be achieved thanthe maximum acceleration of the liquid supply system downwards. If alarge acceleration of the liquid supply system towards the substratetable is detected, the substrate table WT is accelerated away from theliquid supply system to its lowest position where it is safe from theliquid supply system. Also if the substrate table WT suddenly startsaccelerating towards the liquid supply system the liquid supply systemis accelerated with a much larger acceleration away from the substratetable WT. The converse is also true so that the maximum acceleration ofthe liquid supply system is greater than the substrate table in theupwards direction but much lower than that of the substrate table in thedownwards direction.

All of the sensors involved in this control are monitored and processedin hardware which is independent of the normal motion control hardwareand software. If any of the signals from the sensors fails, the liquidsupply system is automatically moved to its upper most position, forexample, by a mechanical spring. This mechanical spring, or a magneticforce for example, also works if there is a power failure to the system.

Precautions are also taken such as only activating the liquid supplysystem if scanning control has started. Furthermore, another situationwhich may arise is that the relative velocity of the liquid supplysystem to the substrate table WT is too high. In this case both theliquid supply system and the substrate table WT are stopped. If therelative velocity is within acceptable limits but the distance betweenthe liquid supply system and the substrate table WT becomes too small,the actuators are also stopped. If the relative velocity and positionare both within acceptable limits, normal operation is allowed.

It may sometimes be necessary to override the safety algorithm, forexample, during attaching of a closure disk as described above. Theclosure disk is positioned on the substrate table WT so that it isnecessary to bring the liquid supply system into close proximity of theclosure disk which requires overriding of the above described safetyalgorithm. It may only be necessary to disable the position check of theabove described safety algorithm but to maintain the velocity check.

FIG. 9 illustrates schematically the control loops for the presentinvention. A liquid supply system 412, of the seal type as describedabove for example, is provided with an actuator system 414 includingthree actuators to enable actuation in the z, Rx and Ry directions. Theactuators may be, for example, Lorentz actuators with permanent magnetsystems used for gravity compensation. The liquid supply system 412 isconstrained by connectors to the base frame BF, reference frame RF ormetrology reference frame MF in the X-Y plane.

An actuator 415 is configured to move the substrate table WT in the Zdirection. The relative positions of the liquid supply system 412 andthe substrate table WT are measured by measuring the relative positionsof the substrate table WT to the metrology reference frame MF (i.e.measuring the distance 418) and between the liquid supply system 412 andthe metrology reference frame MF (i.e. measuring the distance 416). Aprocessor 420 processes this information and supplies it to variousother controllers as described below. The information contains at leastinformation about the relative positions of the liquid supply system 412and the substrate table WT and may also contain other information suchas the distance 418 and/or 416 as well as perhaps the time differentialof any of those distances which equate to the relative velocity of thetwo objects and to the absolute velocity of the substrate table WT andthe liquid supply system 412 respectively.

A damper D and a spring K are schematically illustrated acting betweenthe liquid supply system 412 and substrate table WT. These arerepresentative of the properties of the immersion liquid which transmitsforces between the liquid supply system 412 and the substrate table WT.From a knowledge of the physical properties of the immersion liquid andthe geometry of the liquid in the liquid supply system 412 and thegeometry of the liquid supply system 412 itself, it is possible tocalculate the likely damping coefficient D and spring constant K. Aswill be described below, knowledge of this can be used to either designthe geometry of the liquid supply system 412 to increase the dampingcoefficient D to an extent such that the transmission of forces betweenthe liquid supply system 412 and the substrate table WT is filtered, orto compensate for the damping coefficient D and the spring constant Kwhen actuating the liquid supply system 412 through the actuator 414.

The standard control system for both the actuator 414 for the liquidsupply system 412 and for the actuator 415 for the substrate table WTincludes a positional controller which receives a signal representativeof the desired position of a showerhead of the liquid supply system 412or the substrate table. The positional controllers are labeled 424 and434 for the liquid supply system 412 and the substrate table WT,respectively. Acceleration controllers receive signals representative ofthe desired acceleration of the liquid supply system 412 and/orsubstrate table WT. The acceleration controllers are labeled 422 and432, respectively.

As can be seen from FIG. 9, the positional controllers 424, 423 receivea signal from the processor 420 representative of positional informationregarding substrate table WT and the liquid supply system 412.

Two further elements are provided in the control system. The first ofthese is a filtered feedforward compensator 450 which is equal to theoutput of the liquid supply system positional controller 424, but may besuch a signal which has been filtered to correct for the closed loopcharacteristics of the liquid supply system 412.

The other additional element is a compensator 460 which compensates theoutput of the positional controller 424 and the acceleration controller422 for the stiffness K and damping coefficient D of the immersionliquid between the liquid supply system 412 and substrate table WT. Thiscontroller reduces the forces transmitted between the liquid supplysystem 412 and the substrate table WT due to removal of liquid and gasby the liquid supply system 412. Transmission of these forces can be aproblem with seal type liquid supply systems 412 with a gas seal asdescribed above with respect to FIGS. 4-7.

The present inventors have found that if the input to the actuator 414of the liquid supply system 412 has a low band width (between 10 and 30Hz) and the damping coefficient D is above about 1×10³ N/(m/s), theperformance of the lithography machine can be improved. This is achievedby mechanical design and therefore very cost effective. Calculationshave shown that for an immersion liquid thickness of 0.1 mm the area ofliquid constrained by the liquid supply system on the substrate W shouldbe in the region of 8,000 mm².

In the above description, reference is made to the substrate table WT.This could be the fine positioning upper element of a substrate tableincluded of an upper fine positioning element and a lower coarsepositioning element or a combination of both elements, or of only thecoarse element or any other suitable element of the substratepositioning mechanism of the apparatus.

In an embodiment, there is provided a lithographic projection apparatus,comprising: a substrate table configured to hold a substrate; aprojection system configured to project a patterned beam of radiationonto a target portion of the substrate and having an optical axis; and aliquid supply system configured to provide an immersion liquid on thesubstrate in a space between the projection system and the substrate,wherein at least part of the liquid supply system is free to move in thedirection of the optical axis and/or rotate about at least one axisperpendicular to the optical axis.

In an embodiment, the apparatus further comprises an actuator configuredto adjust at least one of the height and tilt of at least part of theliquid supply system relative to the substrate. In an embodiment, theapparatus further comprises a control system configured to control theactuator to maintain a predetermined height of at least part of theliquid supply system above the substrate. In an embodiment, theapparatus further comprises at least one sensor configured to measure aheight of at least part of the liquid supply system above the surface ofthe substrate, wherein the control system uses a feedback control methodwith input from the at least one sensor. In an embodiment, the apparatusfurther comprises a measurement system configured to measure a surfaceheight of the substrate prior to the entry of the substrate into theprojection system and to store the measured height in a storage device,wherein the control system uses feedforward control with input of themeasured height from the storage device. In an embodiment, the apparatusfurther comprises at least one sensor configured to measure a height ofthe substrate in an exposure position, wherein the control system uses afeedforward control method with input of the height of the substrate inan exposure position. In an embodiment, in a non-actuated state, theactuator positions the at least part of the liquid supply system to amaximum setting away from the surface of the substrate in the directionof the optical axis of the projection system. In an embodiment, theactuator is connected between the at least part of the liquid supplysystem and a base frame that supports the substrate table, or areference frame that supports the projection system, or both the baseframe and reference frame. In an embodiment, a supporting member, or asecond actuator, or both the supporting member and the second actuator,is connected between the at least part of the liquid supply system andthe base frame, or the reference frame, or both the base frame and thereference frame, to keep the at least part of the liquid supply systemsubstantially stationary relative to the projection system in a planeperpendicular to the optical axis. In an embodiment, the actuator ispart of the liquid supply system, the actuator comprising a sealextending along at least part of a boundary of the space between a finalelement of the projection system and the substrate table; and a gas sealconfigured to form a gas seal between the seal and the surface of thesubstrate, wherein the pressure in the gas seal is variable to adjustthe height, or the tilt, or both the height and tilt, of the at leastpart of the liquid supply system with respect to the substrate table. Inan embodiment, the apparatus further comprises at least one sensorconfigured to measure a position of an edge of the liquid relative tothe gas seal and a controller configured to vary the pressure in the gasseal to influence the position of the edge of the liquid. In anembodiment, the controller is configured to operate in a feedforwardmanner, based on the distance between the seal and the substrate. In anembodiment, the predetermined height is approximately 10 μm to 1000 μm.In an embodiment, the apparatus further comprises a dummy diskconfigured to be positioned under the liquid supply system during asubstrate exchange, wherein the dummy disk is attachable to the at leastpart of the liquid supply system, and the at least part of the liquidsupply system is movable away from the substrate with the dummy diskattached during substrate swap. In an embodiment, the part of the liquidsupply system is braced away from the substrate table by anon-electrical, mechanical device or a magnetic device. In anembodiment, the apparatus further comprises a safety controllerconfigured to monitor relative positions, or velocity, or both relativepositions and velocity, of the at least part of the liquid supply systemand the substrate table. In an embodiment, the safety controller isconfigured to control movement of the at least part of the liquid supplysystem, or the substrate table, or both the at least part of the liquidsupply system and the substrate table, in the event of a collision riskbeing determined from the monitoring to prevent the collision. In anembodiment, the apparatus further comprises a positional controllerconfigured to generate control signals to position the at least part ofthe liquid supply system in the direction of the optical axis. In anembodiment, the positional controller is also configured to generatecontrol signals to position the substrate table in the direction of theoptical axis. In an embodiment, the apparatus further comprises afeedforward compensator configured to compensate the control signals toposition the substrate table based on the control signals to positionthe at least part of the liquid supply system. In an embodiment, thefeedforward compensator is configured to compensate for closed loopcharacteristics of the at least part of the liquid supply system. In anembodiment, the apparatus further comprises a damping and stiffnesscompensator configured to compensate the control signals to position theat least part of the liquid supply system to mitigate for the dampingcoefficient and stiffness of immersion liquid between the liquid supplysystem and the substrate. In an embodiment, at least part of the liquidsupply system is free to rotate around axes orthogonal to the opticalaxis.

In an embodiment, there is provided a device manufacturing method,comprising: projecting a patterned beam of radiation onto a targetportion of a layer of radiation-sensitive material on a substrate usinga projection system; providing a liquid on the substrate to fill a spacebetween the substrate and the projection system; and allowing at leastpart of a system which provides the liquid to move freely in thedirection of the optical axis of the projection system and/or rotateabout at least one axis perpendicular to the optical axis.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The description is not intended to limit theinvention.

What is claimed is:
 1. A lithographic projection apparatus comprising: aprojection system configured to project a patterned beam onto a targetportion of a substrate; a movable table, the table at least displaceablewith respect to and under the projection system during projection of thepatterned beam; a structure configured to supply or at least partlycontain liquid, to a space between the projection system and the table,the substrate, or both; an actuator configured to adjust a height of abottom surface of the structure relative to the table, substrate, orboth; and a controller configured to cause the actuator to move thebottom surface to provide, or maintain, a certain height of the bottomsurface with respect to the substrate, substrate table, or both.
 2. Thelithographic apparatus of claim 1, wherein at least a portion of theactuator is part of the structure.
 3. The lithographic apparatus ofclaim 1, wherein the structure comprises a member extending along atleast part of a boundary of the space, the member having the bottomsurface.
 4. The lithographic apparatus of claim 3, wherein at least partof the member is located below the projection system and above thetable, substrate, or both, and the at least part has an aperture toallow the patterned beam to pass through.
 5. The lithographic apparatusof claim 4, wherein the member has an outlet to remove fluid from thespace, the outlet located on the bottom surface.
 6. The lithographicapparatus of claim 5, further comprising an outlet to supply the liquidto above the aperture and above the bottom surface.
 7. The lithographicapparatus of claim 1, further comprising a measurement system configuredto measure the height of the bottom surface.
 8. The lithographicapparatus of claim 1, wherein the actuator comprises a plurality ofactuators, each actuator connected to the structure.
 9. A devicemanufacturing method comprising: projecting a patterned beam, through aliquid, onto a target portion of a substrate, the patterned beamprojected using a projection system and the liquid supplied or at leastpartly contained, by a structure to a space between the projectionsystem and a movable table, the substrate, or both; displacing thesubstrate with respect to and under the projection system duringprojecting of the patterned beam; and causing, using a control system,an actuator to move a bottom surface of the structure relative to thesubstrate, the table, or both, to provide, or maintain, a certain heightof the bottom surface with respect to the substrate, substrate table, orboth.
 10. The method of claim 9, wherein at least a portion of theactuator is part of the structure.
 11. The method of claim 9, whereinthe structure comprises a member extending along at least part of aboundary of the space, the member having the bottom surface and theboundary being smaller than the substrate.
 12. The method of claim 11,wherein at least part of the member is located below the projectionsystem and above the table, substrate, or both, and the at least parthas an aperture to allow the patterned beam to pass through.
 13. Themethod of claim 12, further comprising removing fluid from the spaceusing an outlet located on the bottom surface.
 14. The method of claim13, further supplying the liquid using an outlet above the aperture andabove the bottom surface.
 15. The method of claim 9, further comprisingmeasuring the height of the bottom surface.
 16. The method of claim 9,comprising adjusting the height of the bottom surface using a pluralityof actuators, each actuator connected to structure.
 17. A lithographicprojection apparatus comprising: a projection system configured toproject a patterned beam onto a target portion of a substrate; a movabletable, the table at least displaceable with respect to and under theprojection system during projection of the patterned beam; a structureconfigured to supply or at least partly contain liquid, to a spacebetween the projection system and the table, the substrate, or both; andan electrically-driven actuator configured to move a bottom surface ofthe structure substantially perpendicularly to the table, substrate, orboth.
 18. The lithographic apparatus of claim 17, wherein the structurecomprises a member extending along at least part of a boundary of thespace, the member having the bottom surface, wherein at least part ofthe member is located below the projection system and above the table,substrate, or both, and the at least part has an aperture to allow thepatterned beam to pass through.
 19. The lithographic apparatus of claim18, wherein the member has an outlet to remove fluid from the space, theoutlet located on the bottom surface.
 20. The lithographic apparatus ofclaim 19, further comprising an outlet to supply the liquid to above theaperture and above the bottom surface.
 21. The lithographic apparatus ofclaim 17, further comprising a measurement system configured to measurea value associated with a distance between the structure and the table,the substrate, or both, and a controller configured to cause theactuator to move the bottom surface to provide, or maintain, a certainheight of the bottom surface with respect to the substrate, substratetable, or both, based on the measured value.